Linear accelerator waveguide structures adapted to reduce the phenomenon of pulse shortening



United States Patent LINEAR ACCELERATOR WAVEGUIDE STRUC- TUBES ADAPTED TO REDUCE THE PHE- NOMENUN 0F PULSE SHORTENENG Neil J. Norris, Danville, Roy C. Marker, Orinda, and Richard F. Post, Wainut Creek, (Ialih, assignors, by mesne assignments, to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed June 13, 1960, Ser. No. 35,789 7 Claims. (Cl. 3155.39)

This invention relates to Linear Accelerator Waveguide Structures, and the like, and is particularly directed to such structures wherein the phenomenon of pulse shortening is substantially reduced or eliminated.

The concept and operation of linear electron accelerators of the traveling wave type is well known in the art. Basically such accelerators include one or more accelerating waveguide sections which are essentially uniform in structure, viz., the configuration of the loading elements within each wave guide section is periodic in nature along the total length of the section, although in some applications tapered loading has been employed over a relatively short length near the input to the accelerator.

Each waveguide section is appropriately energized to establish a traveling electromagnetic wave axially therethrough at substantially the velocity of light. Electrons introduced into the section are carried along by the wave and rapidly assume its velocity, approaching the velocity of light. Thereafter, the mass of each electron increases .relativist-ically as the wave carries it through the remainder of the section whereby a high energy electron beam is obtained.

Until recently conventional traveling wave accelerators have been operated at relatively small peak beam currents and short beam pulse lengths, the primary interest having been in the generation of high energy electrons rather than in high beam power. In the past several years, beams of relatively high power have become desirable for certain uses, and accelerators of conventional construction have been designed for operation at higher peak beam currents and longer pulse lengths than previously attempted.

It has been found, however, that when conventional periodic accelerating waveguides are operated at relatively high peak beam currents a phenomenon occurs which materially limits the attainable beam pulse length for a particular peak current, thereby limiting the beam power. More specifically, the puse length of the accelerated beam decreases as the injected current, and therefore the accelerated current, is increased. The effect has been popularly termed pulse shortening, although the same elfect has also been called beam blow-up by some observers in the field.

It is, therefore, a principal subject of our invention to eliminate, or at least substantially reduce, the pulse shortening effect in traveling wave electron acceleration structures.

It is still another object of this invention to provide an accelerating waveguide structure for linear electron accelerators and the like wherein the phenomenon which causes pulse shortening is materially suppressed.

A further object of the present invention is the provision of an accelerating waveguide having a novel nonperiodic disc leading structure which maintains the fundamental mode of the accelerating field throughout the length of the guide and yet attenuates the objectionable mode or other effect productive of pulse shortening.

Yet another object of this invention is to provide an accelerating waveguide having alternate short sections incorporating loading discs or varied spacings compatible 3,222,563 Patented Dec. 7, 1965 with respect to a fundamental accelerating mode but incompatible as to the phenomenon responsible for pulse shortening whereby the phenomenon is not coupled throughout a long enough section to build up to an objectionable degree.

The present invention, together with additional objects and advantages thereof, will become apparent upon consideration of the following description in conjunction with the accompanying drawing wherein the invention is described and illustrated with respect to several preferred embodiments thereof in the interest of clarity, however, no limitations are intended or are to be implied therefrom, reference being made to the appended claims for a precise delineation of the scope of the invention.

In the accompanying drawing: I

FIGURE 1 is a sectional view taken at a diametric plane through a preferred embodiment of an accelerating waveguide structure constructed in accordance with the present invention, and as incorporated in a traveling wave linear electron accelerator; and

FIGURE 2 is a fragmentary sectional view taken at a diametric plane through an alternative embodiment of the waveguide structure.

In considering the waveguide structure of the present invention as illustrated in the drawing and subsequently described in detail herein, it will be of assistance at the outset to first consider the characteristics and probable cause of the pulse shortening phenomenon observed in conventional periodically loaded waveguide structures when operated at high beam power. The observed evidence for the pulse shortening effect shows that the pulse length of the accelerated base in a periodic accelerating waveguide under traveling wave excitation decreases as the injected current, and consequently the accelerated current, is increased.

Through extensive experimentation, we have determined that pulse shortening is not affected by the vacuum conditions in the waveguide, it is not affected by the R.-F. power level in the accelerator, and it is not a simple function, at least, of the injection parameters. In this latter respect, the important factors in the pulse lengthpeak current relationship are not the injection voltage or injected current, but rather the accelerated current and pulse length. More specifically, we have observed that the trailing edge of an accelerated beam pulse collected in a target at the end of the accelerating waveguide drops suddenly at a time which is not limited either by the injection pulse or the R.-F. driving pulse.

At peak currents just at the threshold of the current which causes a decrease in pulse length, there is considerable pulse length variation with some of the pulses being pulse shortened and others not. As the current is increased beyond the threshold, however, and the pulse length becomes even shorter, the trailing edge of the pulse becomes sharp and the pulse length becomes very steady, indicating that there is uncertainty or variation in the time of the onset of the pulse shortening phenomenon.

Many experiments were also performed to determine the direction and type of deflection of the electrons missing from the pulse shortened beam pulses. All of these experiments, and particularly magnetic steering experiments in which it was possible to steer the part of the beam missing from a normal pulse through the aperture in the end of the waveguide, indicated that the loss of beam is caused by a radial blow-up, viz., divergence of the beam in the waveguide. The steering experiments further indicated that the beam diverges radially and uniformly in all directions and that this radial blow-up occurs near the beginning of the waveguide section.

We have also observed that there is considerable difference in the pulse shortening effect in waveguide sections of similar construction but different. lengths. The

shorter the section, the higher is the peak current possible with a particular pulse length. In addition, in waveguide sections of equal length but varied parameters of disc spacing and accelerating tube diameter, the pulse shortening effect varies considerably.

Where the disc spacing is such as to define fewer cavities along a given length of waveguide section, the pulse shortening is less pronounced. The pulse shortening is also less pronounced at increased accelerating tube diameters. From the foregoing, we have concluded that the magnitude of the pulse shortening effect is governed by the number of cavities coupled to the beam and the beam-cavity coupling coefficient, a function of the beam radius.

From the preceding experimental observations, we feel that the most logical cause of the pulse shortening phenomenon is a mode which is set up in the accelerating waveguide by the action of the beam itself, the phenomenon being sensitive to the amount of accelerated beam. This consideration appears likely inasmuch as the accelerated beam is bunched to a very high degree with a very high density of electrons at one phase in every wavelength along the guide.

It is well known that a bunched beam of this type may be used to excite any waveguide structure which is resonant to its fundamental frequency or to one of the many harmonics which are contained in this very tightly bunched beam. Moreover, waves of this character which are excited in a periodic structure will travel along this structure either in the forward or backward direction according to their phase and group velocities. Hence we visualize the accelerated beam exciting one or mode modes in the first few cavities of the accelerating waveguide which travel from their point of excitation all along the guide back toward the front end of the guide.

If one of the excited modes is of such a type that it establishes an outwardly directed radial component of force acting on the electrons of the beam, then the electrons which are at the axis of the waveguide would be deflected in such a way that they would not come out through the limited aperture at the output of the struc ture.

In order for the observed pulse to be produced by such a deflection of the beam electrons, the size of the field induced by the mode would have to be appreciable compared to the main field. It is believed that in order for this field to be of sufficient proportions to cause a sudden divergence of the beam, there must be a feedback mechanism involved Which makes the excitation of the mode dependent upon the radial component of the electron velocity. An electron beam that started to diverge from a point would hence couple more power into the exciting mode and this in turn would increase the amount of deflection of the beam and cause the mode to very suddenly deflect all of the electrons away from the axis of the accelerator guide.

Such a build up of mode phenomenon would likely depend upon the length of Waveguide section in which the mode can be excited. Therefore, the foregoing mode build-up assumption appears logical in view of the observed dependence of the pulse shortening effect on the length of a given waveguide section, number of cavities coupled to the beam, beam-cavity coupling coefficient, peak current accelerated, and duration of the current pulse.

Based upon this picture of the cause of pulse shortening being an objectionable beam excited driven regenerative mode and our observation of the phenomenon, we have provided an accelerating waveguide structure which eliminates or materially reduces the pulse shortening effect.

Referring to the drawing, the accelerating waveguide structure is seen to broadly comprise a plurality of short sections of different characteristics so as to be not continuously periodic along the entire length of the structure. The varied characteristics of the short sections are compatible with coupling of the fundamental accelerating mode along the entire length of the guide at a phase velocity within close tolerance of the velocity of light but are incompatible with coupling of higher order modes between sections. Accordingly, it is felt that the resultant minimization in pulse shortening in this waveguide structure arises by virtue of the fundamental accelerating mode being maintained through the waveguide while the objectional higher order mode is not coupled throughout a long enough section to build up due to the varied characteristics of the short sections.

Considering now the waveguides structure in greater detail relative to the preferred embodiment illustrated in FIGURE 1, the numeral 11 designates generally a microwave linear electron accelerator which includes at least one accelerating waveguide 12 in accordance with the present invention. Electrons are injected into the waveguide at a velocity substantially less than that of light by means of an electron gun 13, or the like, coupled to the input end of the waveguide.

A source of microwave energy such as a microwave oscillator 14 is also coupled to the input end of the guide to establish a traveling electromagnetic wave having a fundamental or accelerating mode of propagation through the guide at a phase velocity within close tolerance of the velocity of light. The wave energy reaching the output end of the guide is usually absorbed by means of a load 16 coupled thereto.

Electrons injected into the waveguide from electron gun 13, in the proper phase relative to the traveling wave, are trapped and carried along by the wave to form tight bunches which rapidly assume a velocity sub stantially equal to that of light. Thereafter the bunched electron beam continues to gain energy as it travels through the waveguide but without any appreciable change in velocity, the increased energy taking the form of an increase in the electrons mass. The foregoing operation is typical in conventional linear electron accelerators and hence further detail relative thereto is not presented herein. Moreover, various well-known departures may be made from the overall accelerator arrangement illustrated in the drawing, and it is not intended that the novel waveguide 12 be in any way limited to incorporation in a specific accelerator arrangement.

For example, a buncher section may be provided between the electron gun and waveguide in a well known manner without departing from the scope of the present invention.

It will be noted that, as regards the novel waveguide 12 of the present invention which facilitates operation of the accelerator 11, or any other electron accelerator in which it is incorporated, at high beam power by materially reducing or eliminating pulse shortening, in the instant embodiment the short length sections of different characteristics are provided by varied spacing of the loading discs 17 in alternate sections 18 of the cylindrical guide envelope 19. More specifically, each section 18 has a length which is preferably predetermined to correspond to one wave length of the traveling wave established in the guide, as by means of oscillator 14 at a given operating frequency.

In the drawing, alternate ones of these sections are designated as 18 and 18", respectively, and the loading discs in these sections are correspondingly designated 17' and 17".

The loading discs 17' in each section 18 are longitudinally spaced to define therebetween four cells 21 per section whereas the loading discs 17" in each alternate section 18" are spaced to define three cells 21" per section. Hence the cells 21', 21" of alternate sections 18', 18" are respectively one-fourth and one-third wavelength long. It can be shown that the fundamental mode for accelerating electrons in these alternate sections of onefourth and one-third wavelength disc spacing is similar and hence the fundamental mode is coupled throughout the length of waveguide 12. The electron accelerating field is thus maintained along the entire length of the waveguide. However, the alternate sections 18', 18" by virtue of the varied disc spacings therein are not compatible with respect to higher order modes. Accordingly, any coupling of such higher order modes between alternate sections is of extremely low order or eliminated.

In other words, the propagation of the higher order modes from one end of the waveguide to the other is substantially prevented by virtue of the incompatible characteristics of the alternate sections. It is therefore thought that the substantial reduction in pulse shortening of the electron beam in an accelerator employing the waveguide 12, such as the accelerator 11, when operated for highbeam power is attributable to a reduction in propagation of the probably existent objectionable beam excited mode throughout the guide whereby such mode cannot build up.

Although a specific spacing of the loading disc and number of cells per section are employed in the alternate sections 18', 18 of the preferred embodiment of the waveguide 12 described heretofore, it will be appreciated that a variety of other specific configurations are possible. For example, integral multiples of the number of cells and corresponding quotients of the disc spacing of the alternate four-three cells per wavelength configuration may as well be advantageously utilized. For instance, alternate sections of eight and six cells per section could as well be employed.

Furthermore the succession of sections need not alternate in twos, an alternating succession of three or more sections of dissimilar characteristics being also contemplated by the present invention. In this regard a waveguide with alternating successions of sections having respectively five, four, and three cells per section might be employed to eliminate the pulse shortening effect. In any of the various possible configurations of non-periodic waveguides in accordance with the invention it would, of course, be appreciated that no matter what specific number of sections of different characteristics and specific number of cells per section are employed to render the waveguide non-periodic, the sections must be mutually compatible with respect to correct coupling therethrough of a fundamental mode capable of accelerating electrons within close tolerance of the velocity of light.

As an example of one other configuration of a nonperiodic waveguide structure which is compatible with respect to a fundamental electron accelerating mode and incompatible with respect to higher order modes, reference is made to the alternative embodiment of FIGURE 2. As shown therein, a waveguide 22 is provided having alternate one wavelength long sections 23', 23", wherein the spacing of the leading discs 24', 24" is such as to define respectively three and two cells 26, 26 per section.

It can be shown that the fundamental modes in these sections 23', 23" are similar and capable of accelerating electrons such that an electron accelerating field is maintained throughout the length of the guide. Higher order modes, moreover, are not coupled between alternate sections with sufficient magnitude to build up. The reduction in pulse shortening attainable with waveguide 22 is accordingly believed to be due to the attenuation or elimination of an objectionable mode therein, as in the case of the embodiment of FIGURE 1.

Integral multiples of the numbers of cells per alternate sect-ions 23', 23" are also suitable to the ends of the present invention.

While the salient features of this invention have been described in detail with respect to certain embodiments and modifications thereof, it will of course be apparent that further modifications may be made within the spirit and scope of the invention, and it is not desired, therefore, to limit the invention except insofar as defined in the following claims.

We claim:

1. An accelerating waveguide structure for electrons comprising a plurality of sections of periodically loaded waveguide disposed in electrically coupled axially aligned relation with the periodic loadings of sets of alternate sections differing from each other to sustain varied modes of electromagnetic wave propagation therein, said alternate sections having loadings mutually compatible with the coupling of a fundamental electron accelerating mode of electromagnetic wave propagation therethrough at a phase velocity substantially equal to the velocity of light while being incompatible with the coupling of higher order modes of electromagnetic Wave propagation between sections.

2. In a travelling wave linear electron accelerator including at least an electron gun and a source of microwave driving energy, the combination comprising an elongated waveguide envelope for connection at its input end to said electron gun and source of microwave driving energy, and a plurality of loading discs mounted within the envelope with varied uniform axial spacing between the discs in alternate wavelength sections thereof, said spacing between discs in alternate sections being compatible with coupling of a fundamental accelerating electron mode at a phase velocity substantially equal to the velocity of light while being incompatible with coupling of higher order modes therebetween.

3. An electron accelerating waveguide structure comprising an elongated waveguide envelope, and a plurality of loading discs mounted within the envelope and uniformly axially spaced in alternate wavelength sections of the envelope to define respectively four and three cells per section between the discs.

4. An electron accelerating Waveguide structure according to claim 3, but wherein the discs in alternate wavelength sections of the envelope are spaced to define respectively an integral multiple of four and three cells per section.

5. An electron accelerating waveguide structure comprising an elongated waveguide envelope, and a plurality of loading discs secured within the envelope and uniformly axially spaced in alternate wavelength sections of the envelope to define respectively three and two cells per section between the discs.

6. An electron accelerating waveguide structure according to claim 5, but wherein the discs in alternate Wavelength sections of the envelope are spaced to define respectively an integral multiple of three and two cells per section.

7. An electron accelerating waveguide structure comprising an elongated waveguide envelope, and a plurality of loading discs secured within the envelope and disposed in alternating successions of wavelength long sections with each section having varied uniform spacings between the discs mutually compatible with the coupling of a fundamental electron accelerating mode of electromagnetic wave propagation therethrough.

References Cited by the Examiner UNITED STATES PATENTS 2,653,271 9/1953 Woodyard 3155.42 2,880,353 3/1959 Warnecke et al. 3l55.41 X 2,888,596 5/1959 Rudenberg 3155.4l 2,908,844 10/1959 Quate 3153.6 3,070,726 12/ 1962 Mallory 315-5.42

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS, Examiners. 

1. AN ACCELERATING WAVEGUIDE STRUCTURE FOR ELECTRONS COMPRISING A PLURALITY OF SECTIONS OF PERIODICALLY LOADED WAVEGUIDE DISPOSED IN ELECTRICALLY COUPLED AXIALLY ALIGNED RELATION WITH THE PERIODIC LOADINGS OF LSETS OF ALTERNATE SECTIONS DIFFERING FROM EACH OTHER TO SUSTAIN VARIED MODES OF ELECTROMAGNETIC WAVE PROPAGATION THEREIN, SAID ALTERNATE SECTIONS HAVING LOADINGS MUTUALLY COMPATIBLE WITH THE COUPLING OF A FUNDAMENTAL ELECTRON ACCELERATING MODE OF ELECTROMAGNETIC WAVE PROPAGATION THERETHROUGH AT A PHASE VELOCITY SUBSTANTIALLY EQUAL TO THE VELOCITY OF LIGHT WBILE BEING INCOMPATIBLE WITH THE COUPLING OF HIGHER ORDER MODES OF ELECTROMAGNETIC WAVE PROPAGATION BETWEEN SECTIONS. 